Mutations in SLC6A19, encoding B0AT1, cause Hartnup disorder

Abstract

Hartnup disorder, an autosomal recessive defect named after an English family described in 1956 (ref. 1), results from impaired transport of neutral amino acids across epithelial cells in renal proximal tubules and intestinal mucosa. Symptoms include transient manifestations of pellagra (rashes), cerebellar ataxia and psychosis 1,2 . Using homozygosity mapping in the original family in whom Hartnup disorder was discovered, we confirmed that the critical region for one causative gene was located on chromosome 5p15 (ref. 3). This region is homologous to the area of mouse chromosome 13 that encodes the sodium-dependent amino acid transporter B 0 AT1 (ref. 4). We isolated the human homolog of B 0 AT1, called SLC6A19, and determined its size and molecular organization. We then identified mutations in SLC6A19 in members of the original family in whom Hartnup disorder was discovered and of three Japanese families. The protein product of SLC6A19, the Hartnup transporter, is expressed primarily in intestine and renal proximal tubule and functions as a neutral amino acid transporter.Despite molecular characterization of other proximal tubule transporters, the neutral amino acid carrier defective in Hartnup disorder (OMIM 2345000) has resisted genetic identification 2 . We carried out homozygosity mapping and fine mapping in ten members of two consanguineous families (the siblings in whom Hartnup disorder was originally discovered 1 ; family A; Fig. 1a) and in siblings from the US 5 (family B; Fig. 1a). We found linkage of Hartnup disorder to 5p15 only in family A, with a maximum combined multipoint lod score of 2.31 at 11.24 cM (P ¼ 0.01). This confirmed our previous results showing linkage to chromosome 5p15 (ref.3). In family B, we obtained a maximum multipoint lod score of À2.40 at 15.81 cM.We simultaneously pursued two mouse monoamine transporterrelated orphan genes, Slc6a18 (also called Xtrp2; ref. 6) and Slc6a19 (encoding B 0 AT1; ref. 4). These members of the SLC6 family of transporters map to the mouse chromosomal region that is homologous to human chromosome 5p15. Both Slc6a18 and Slc6a19 showed abundant expression in mouse kidney, as assessed by real time RT-PCR (Fig. 2a). Immunohistochemistry confirmed expression of mouse B 0 AT1 at the brush border of small intestine (data not shown) and kidney proximal tubule cells (Fig. 2b).The human homolog, B 0 AT1, is encoded by the predicted locus SLC6A19, with a 2,022-bp open reading frame. PCR amplification using human kidney cDNA produced a 1,905-bp product with 100% identity to SLC6A19 sequence. We next determined the genomic organization of SLC6A19, which has a stop codon 28 bases before the ATG in the 5¢ untranslated region. SLC6A19 has 12 coding exons. The B 0 AT1 protein contains 634 amino acids and 12 predicted transmembrane regions (Fig. 1b). In a panel of human cDNAs, we detected robust expression of SLC6A19 in kidney and small intestine, with minimal expression in pancreas (Fig. 2c). SLC6A19 was also expressed in stomach, liver, duodenum and ileocecum (data n...

Characterization of mouse amino acid transporter B0AT1 (slc6a19)

“…Furthermore, we and others [21,22] subsequently demonstrated that SLC6A19 constitutes a dominant transport activity for neutral amino acids in the kidney and intestine by showing that a malfunction of SLC6A19 results in severe neutral aminoaciduria, known as Hartnup disorder. Mouse slc6a19 is expressed in the outer cortex of the kidney and in the intestine [10], and therefore most likely corresponds to the transporter characterized in vesicle studies using kidney cortex or intestinal preparations.…”

“…We and others [21,22] have shown recently that mutations in the human B 0 AT1 (SLC6A19) cause Hartnup disorder. This indicates that B 0 AT1 is a major transport activity for neutral amino acids in the kidney and intestine.…”

“…Thirdly, the substrate K 0.5 value decreases with increasing Na + concentration. Fourthly, in situ hybridization [10] and immunohistochemistry [22] demonstrate expression of the transporter in the convoluted part of the proximal tubule (S1 and S2), but not in the pars recta (S3). Fifthly, the transporter is localized in the brush-border membrane on the apical side [22].…”

“…Mouse slc6a19 is expressed in the outer cortex of the kidney and in the intestine [10], and therefore most likely corresponds to the transporter characterized in vesicle studies using kidney cortex or intestinal preparations. However, expression of SLC6A19 has also been reported in the S3 segment [22].…”

Novel Mutation in SLC6A19 Causing Late-Onset Seizures in Hartnup Disorder

“…SLC6A19 encodes the amino acid transporter B 0 AT1, which cotransports Na + and a broad range of neutral amino acids from the luminal compartment into the cells [4]. To date, about 20 mutations in SLC6A19, including missense, splicing, and small deletions and insertions, have been reported [5][6][7]. We recently examined a Korean boy with Hartnup disorder with novel mutations of the SLC6A19 gene.…”